This project will investigate muscular fatigue as a causative factor in low back injuries with the long-term objective of better evaluating, treating, and preventing low back disabilities. The aims of this project are: (1) during a continuous, repetitive, fatiguing lifting task, to describe the changes which occur in the instantaneous fatigue state of muscles of the low back and in selected kinematic and kinetic features of lifting; and (2) during this same lifting task, to determine the relationship between fatiguability and the fiber composition of the muscles of the low back. Thirty normal subjects will undergo a screening medical examination, and lift strength testing. Under direction of computer controlled visual and auditory cues, each subject will then work at a repetitive lifting task having load and frequency parameters which are known (from psychophysical studies) to be fatiguing. During repetitive lifting the following variables will be monitored: (1) flexion/extension motion of the lumbar spine; (2) vertical acceleration of the load; and (3) myoelctrical activity from five locations over the paraspinal muscles. The task will continue until predetermined fatigue criteria are reached (or the subject requests to discontinue the experiment). Following testing, fiber composition of the low back muscles will be determined by muscle biopsy. Data analysis will consist of determining the relationships among the variables as stated in the specific aims.